Inexpensive photodetectors that measure the intensity of light in a number of wavelength bands are required in a number of devices. For example, light sources that utilize red, blue, and green LEDs to generate light that is perceived as being a particular color often utilize photodetectors in a servo loop that maintains the output of the LEDs at predetermined levels to compensate for aging. The photodetectors are used to measure the output of each LED. A controller adjusts the average current to each LED such that the measured outputs are maintained at target values determined by the perceived color of light that is to be generated.
In one commonly used type of photodetector, the photodetector utilizes photodiodes that are covered by pigment filters that limit the response of each of the photodiodes to light in a corresponding band of wavelengths. The signals from the various photodiodes are processed to provide signals that represent the output of each of the LEDs. The signal from each photodiode is determined by the incident light, the bandpass filter characteristics of the pigment and various background signals that are present independent of the intensity level of the light reaching the photodiode. The light-independent signals are often referred to as the “dark current”. The errors generated by the dark current can be significant in a number of applications; hence, schemes for correcting for the dark current have been developed. In addition, removing the contributions to the final signals that result from the dark current improves the dynamic range of the photodetector, and hence, the photodetector can be used to control the LEDs over a larger range of light intensities.
In one dark current correction scheme, the errors generated by the dark current are removed by measuring the output of the photodiode when no light is present and then subtracting the measured signal value from the signals generated by the photodiode in the presence of light. In this arrangement, the photodiodes are identical in structure and differ only in the type of pigment filter that overlies each photodiode. An additional photodiode that is covered by an opaque layer that blocks all light is included in the photodetector. The signal from this photodiode is then subtracted from that generated by the photodiodes that are covered with the various pigment filters. This scheme, however, significantly increases the cost of the photodetectors, since additional masking steps are needed to provide the opaque layer over the additional photodiode.